text: Flange fittings are generally used to connect valves, inline instruments or equipment nozzles. Two surfaces are joined tightly together with threaded bolts, wedges, clamps, or other means of applying high compressive force. Although a gasket, packing, or O-ring may be installed between the flanges to prevent leakage, it is sometimes possible to use only a special grease or nothing at all (if the mating surfaces are sufficiently precisely formed). Although flange fittings are bulky, they perform well in demanding applications such as large water supply networks and hydroelectric systems. 
  Flanges are rated at 150, 300, 400, 600, 900, 1500, and 2500 psi; or 10, 15, 25, 40, 64, 100, and 150 bars of pressure. Various types of flanges are available, depending on construction. Flanges used in piping (orifice, threaded, slip-on, blind, weld neck, socket weld, lap-joint, and reducing) are available with a variety of facings, such as raised, flat, and ring-joint.
  Flange connections tend to be expensive because they require the precision forming of metal. Factory-installed flanges must meet carefully measured dimensional specifications, and pipe segments cut to length on-site require skilled precision welding to attach flanges under more-difficult field conditions.